U.S. Department of Energy officials announced Thursday they will use only containers that can pass a "dynamic crush test" to transport nuclear materials from the former nuclear weapons plant 13 miles northwest of Denver. They decided not to invoke a "national security exemption" allowing them to use larger, weaker containers, called DT-22s.

The DT-22 container, according to its designer, will not pass a crush test in which a 1,100-pound weight is dropped on it from 30 feet, simulating a high-speed crash.

Use of the containers had been challenged in lawsuits by South Carolina Gov. Jim Hodges seeking to block shipments to the Savannah River Site near Aiken, S.C., and by activist neighbors of the Lawrence Livermore National Laboratory near San Francisco.

"We want to move forward, rather than engage in unnecessary and costly litigation from environmental groups that could delay the environmental restoration of our Rocky Flats facility," Assistant Energy Secretary Jessie Roberson said in a statement.

Department of Energy engineers had raised numerous objections to using the weaker containers, according to e-mails obtained by the California activists through a Freedom of Information Act request. The engineers said the exemption was not for national security, but to cut costs and meet the politically popular 2006 deadline, despite mistakes by the department and the cleanup contractor, Kaiser-Hill. But political appointees in the Clinton administration granted the exemption anyway.

"If an SST (truck carrying nuclear material) was hit by a train, the crush environment would occur," one engineer wrote. "If an SST was hit from behind by a large, heavy vehicle, the crush environment may occur."

Energy officials will have to cut some classified nuclear weapons pieces down to size to fit into the smaller containers, which are about the size of two office trash cans stacked on top of each other. Those who supported using the DT-22s, which are 45-gallon drums, said cutting the pieces up would increase risks to workers at the plant and cost up to $100 million more. DOE spokesman Joe Davis did not return a telephone call for comment Thursday.

Rocky Flats produced explosive plutonium "triggers" for 40 years until 1989. The federal government is spending $7 billion to turn it into a wildlife refuge.

DOE officials say shipments must begin soon to meet the 2006 cleanup deadline. But they've been delayed until at least June 15 by Hodges' lawsuit.

A DOE report issued Thursday indicated that Hodges might not be the only delay. A new Energy Department report says that Rocky Flats is behind schedule in getting containers filled and ready for shipment.

The packaging was supposed to be done by the end of this month, but the DOE inspector general says that only about half the 1,900 containers will be ready by then. They are expected to be finished by early next year.

"Missed milestones increase the risk of delays to the planned 2006 Rocky Flats closure date," wrote Inspector General Gregory Friedman, "which has significant cost implications for the department's environmental remediation effort."

Kaiser-Hill Co., the contractor doing the cleanup at Rocky Flats, says the plant will be cleaned up and closed by Dec. 15, 2006. Company spokesman John Corsi said the packaging machine did not work well early on, but is now in working order.

"The rates have varied, but we're confident with the rate we're at now," Corsi said. "It's not going to impact 2006."